Spring regulated variable flow electric water pump

ABSTRACT

An electric water pump having a motor with an axially moveable rotor unit. A rotary pump member is fixed for axial movement with the rotor unit to vary its position within a pump chamber so as to vary the flow rate through the pump chamber.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/140,854 filed Mar. 31, 2015. The entire disclosure of the aboveapplication is incorporated herein by reference.

FIELD

The present disclosure relates generally to water pumps for motorvehicles. More specifically, the present disclosure relates to avariable flow electric water pump equipped with an axially-moveablerotor/impeller assembly.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

As is well known, water pumps are typically used in motor vehicles aspart of a thermal management system for pumping a liquid coolant tofacilitate heat transfer between the coolant and the internal combustionengine during vehicle warm-up and operation. Most commonly, acentrifugal water pump having a rotary pump member, such as an impeller,is configured to draw the coolant into an axial inlet and discharge thecoolant through a radial discharge outlet. In many vehiculararrangements, the impeller is fixed to an impeller shaft that isrotatably driven (via an accessory drive system) by the crankshaft ofthe engine. Thus, the impeller speed is directly proportional to theengine speed. To provide a variable flow feature to such shaft-drivenwater pumps, it is known to permit limited axial displacement of theimpeller within the pump chamber. For example, U.S. Pat. No. 7,789,049discloses a water pump having an axially-moveable impeller that isspline mounted to the engine-driven shaft, and an electromagneticactuator operable to control axial movement of the impeller betweenextended and retracted positions along the shaft so as to variablyregulate the fluid flow characteristic between the fluid inlet and thedischarge outlet. Similarly, U.S. Pat. No. 5,800,120 discloses a waterpump having a shaft-driven impeller equipped with axially-moveableblades, the position of which is controlled via a hydraulic actuator.

It is also well known to install an auxiliary water pump, such as anelectric water pump, in the engine coolant system to provide augmentedcontrol over the fluid flow. Generally, electric water pumps include anelectric motor having a stationary stator and a rotor that is drivinglycoupled to the impeller. Examples of electric water pumps are disclosedin commonly-owned U.S. Publication No. US2013/0259720 titled “ElectricWater Pump With Stator Cooling” and U.S. Publication No. US2014/0017073titled “Submerged Rotor Electric Water Pump with Structural Wetsleeve”,the entire disclosures of which are incorporated herein by reference.One drawback associated with many conventional electric water pumps isthe need to provide a rotor encoder or another type of speed sensorwithin the electric motor to assist in accurate low speed (i.e. lessthan 600 RPM) pump control via a closed loop motor control system.Additionally, a need exists to provide variable flow at such low speedsthat is not directly proportional to motor speed in an effort to meetcustomer expectations.

In view of the above, a need exists in the art to design and developsimplified and low-cost electric water pumps capable of providingvariable flow characteristics and which can be easily substituted forotherwise conventional electric water pumps in motor vehicleapplications.

SUMMARY

This section provides a general summary of the disclosure and is notintended to act as a comprehensive and exhaustive disclosure of its fullscope or all of its features, advantages, objectives and aspects.

It is an objective of the present disclosure to provide an electricwater pump that meets the above-identified needs and provides atechnological advancement over conventional electric water pumps.

It is another objective of the present disclosure to provide an electricwater pump equipped with an electric motor having a stationary statorassembly and an axially-moveable rotor unit adapted to cause concurrentaxial movement of a rotary pump member within a pump chamber forvariably regulating fluid flow between an inlet and an outletcommunicating with the pump chamber.

It is similar objective of the present disclosure to provide an electricwater pump having a rotor/impeller assembly that is axially moveablerelative to a stationary stator assembly for varying the size of aclearance gap between a volute in the pump chamber and the impeller.

It is a related objective of the present disclosure to control movementof the rotor/impeller assembly so as to provide a low flow output at lowrotor speeds and a high flow output at high rotor speeds. In thisregard, the rotor/impeller assembly is located in a low flow positionrelative to the stator assembly when rotated at low rotor speeds and ina high flow position relative to the stator assembly when rotated athigh rotor speeds.

In accordance with a first embodiment of an electric water pumpconstructed and functional in accordance with the objectives of thepresent disclosure, the rotor/impeller assembly is normally biasedtoward its low flow position by a mechanical biasing arrangementdisposed between the rotor unit and a stationary member within a pumphousing. Movement of the rotor/impeller assembly from its low flowposition toward its high flow position is a result of a pressuredifferential (AP) generated between upper (i.e. outer) and lower (i.e.inner) portions of the impeller and which is a function of the rotaryspeed of the rotor/impeller assembly.

In accordance with a second embodiment of an electric water pumpconstructed and functional in accordance with the objectives of thepresent disclosure, the rotor/impeller assembly is normally located inits low flow position by a magnetic biasing arrangement provided by anaxially-offset magnetic field between the stator assembly and the rotorunit that is established by rotor magnets having an increased length inthe direction of the impeller so as to provide a centering relationshipwith the stator assembly during low speed operation.

The present disclosure is directed to a variable flow electric waterpump for use in an engine coolant system of a motor vehicle comprising:a pump housing defining a fluid chamber and a motor chamber, the fluidchamber including a fluid inlet and a discharge outlet for providing aflow of a coolant through the fluid chamber; an electric motor disposedin the motor chamber and including a stationary stator assembly and arotor unit having a rotor shaft supported for rotation about alongitudinal axis and at least partially extending into the fluidchamber; an impeller fixed for rotation with the rotor shaft anddisposed within the fluid chamber and being operable to pump the coolantfrom the fluid inlet to the discharge outlet; and a biasing arrangementoperable for normally locating the rotor unit in a first positionaxially offset relative to the stator assembly for locating the impellerin a retracted position within the fluid chamber so as to provide a lowflow characteristic between the fluid inlet and the discharge outletwhen the impeller is rotatable driven by the rotor shaft at a lowimpeller speed.

The variable flow electric water pump of the present disclosure isfurther operable when the impeller is rotatably driven at a higherimpeller speed to forcibly move the impeller to an extended positionwithin the fluid chamber, in opposition to the preload exerted bybiasing arrangement, for causing the rotor unit to be located in asecond position axially aligned with the stator assembly.

The variable flow electric water pump of the present disclosure can beequipped with a mechanical biasing arrangement configured to normallyexert a biasing force on the rotor unit selected to bias the rotor unittoward its first position. The mechanical biasing arrangement caninclude a mechanical biasing member, such as one or more spring members,disposed between an upper portion of the rotor unit and a stationarymember or portion of the pump housing.

The variable flow electric water pump of the present disclosure canoptionally be equipped with a magnetic biasing arrangement configured tonormally locate the rotor unit in its first position.

The variable flow electric water pump of the present disclosure canfurther include an interface formed in the pump housing between thefluid inlet and the discharge outlet defining a flange surface. Theimpeller can be configured to include an outer rim surfaced aligned withthe flange surface such that a first larger clearance gap is establishedtherebetween when the impeller is located in its retracted position. Thefirst larger clearance gap functions to establish a low flowcharacteristic when the impeller is driven at the low impeller speeds bythe electric motor. In contrast, a second smaller clearance gap isestablished when the impeller is located in its extended position so asto create a high flow characteristic when the impeller is driven by theelectric motor at the high impeller speeds.

Further areas of applicability will become apparent from the detaileddescription provided herein. As noted, the description of theobjectives, aspects, features and specific embodiments disclosed in thissummary are intended for purposes of illustration only and are notintended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations and, as such,are not intended to limit the scope of the present disclosure.

FIG. 1 is a sectional view of a variable flow electric water pumpconstructed in accordance with a first embodiment of the presentdisclosure to include a mechanically-biased rotor/impeller assemblywhich is shown located in a first or low flow position relative to astationary stator assembly;

FIG. 2 is another sectional view of the variable flow electric waterpump shown in FIG. 1 now illustrating the spring-biased rotor/impellerassembly located in a second or high flow position relative to thestator assembly;

FIG. 3 is a graph illustrating the low-speed flow characteristicsprovided by the variable flow electric water pump shown in FIGS. 1 and 2in comparison to a conventional fixed flow electric water pump;

FIG. 4 is a sectional view of a variable flow electric water pumpconstructed in accordance with a second embodiment of the presentdisclosure to include a magnetically-biased rotor/impeller assemblywhich is shown located in a first or low flow position relative to thestationary stator assembly;

FIG. 5 is another sectional view of the variable flow electric waterpump shown in FIG. 4 now illustrating the rotor/impeller assemblylocated in a second or high flow position relative to the statorassembly; and

FIGS. 6A and 6B are a partial sectional view of a slightly modifiedversion of the variable flow electric water pump of FIGS. 1 and 2.

Corresponding reference numerals indicate corresponding componentsthroughout the several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be more fully describe with reference tothe accompanying drawings. However, the following description is merelyexemplary in nature and is not intended to limit the present disclosure,its subject matter, applications or uses. To this end, exampleembodiments of an electric water pump are provided so that thisdisclosure will be thorough and will fully convey the scope to thoseskilled in this art. Numerous specific details are set forth, such asexamples of specific components, devices and methods to provide athorough understanding of the embodiments in many different forms, andsuch should not be construed to limit the intended scope of protectionafforded by this disclosure. As is understood, some well-knownprocesses, structures and technologies are not described in detailherein in view of the understanding afforded thereto by those skilled inthis art.

In general, the present disclosure relates to an electric pump and, moreparticularly, to an electric water pump of the type applicable andwell-suited for use and installation in motor vehicles for pumping aliquid coolant through an engine cooling system. However, the teachingsprovided herein are considered to be adaptable to any other electricpump required to move a medium (i.e. air, water, coolant, oil, etc.)within a pumping system requiring a variable flow capability.

With particular reference to FIGS. 1 and 2 of the drawings, an electricwater pump 10 constructed and functional in accordance with a firstexample embodiment of the present disclosure will now be described ingreater detail. Pump 10 generally includes a pump housing 12, anelectric motor 14, and a pump unit 16. Pump housing 12 is shown in thisnon-limiting example to include a cylindrical outer housing 18, a firstor bottom cap 20, and a second or top cap 22. Outer housing 18 isgenerally cup-shaped and includes an open end section 24 to which bottomcap 20 is secured, and an end plate section 26 to which top cap 22 issecured. End plate section 26 of outer housing 18 is formed to define araised annular rim 28 extending from a planar mounting surface 30. Acentral pump pocket 32 is formed in rim 28 and is aligned on thelongitudinal axis “A” of pump 10. A pair of internal annular bosses 34and 36 also extend from end plate section 26 of outer housing 18 and arealigned with the longitudinal axis. A thorough bore 38 extends betweenpump pocket 32 and a bearing pocket 40 associated with annular boss 34.

Bottom cap 20 is configured, in this non-limiting example, to include anannular rim 44 extending from a planar mounting surface 46, and anelongated cylindrical hub 48, both of which are concentric with thelongitudinal axis. End section 24 of outer housing 18 includes an innerdiameter wall surface 50 configured to be pressed against an outerdiameter surface 52 of annular rim 44. End section 24 also includes aplanar end surface 54 configured to engage mounting surface 46 on bottomcap 20. While not specifically shown, a suitable fastening arrangementis provided to secure bottom cap 20 to outer housing 18 so as to definean internal motor chamber 56. A blind bore 58 is formed in hub 48 andfurther defines a bearing pocket 60.

Top cap 22 is shown, in this non-limiting example, configured to includean axially-extending tubular section 64 defining a fluid inlet 66, aradially-extending tubular section 68 defining a fluid discharge outlet70, and a volute section 72 defining an impeller cavity 74 in fluidcommunication with fluid inlet 66 and discharge outlet 70. An interface76 is formed in top cap 22 between fluid inlet 66 and impeller cavity 74and includes a first flange surface 78 and a second flange surface 80.Top cap 22 includes a stepped flange section 82 configured to enclose aportion of raised rim 28 on end plate section 26 of outer housing 18.Top cap 22 also includes a planar inner mounting surface 84 configuredto engage outer mounting surface 30 on outer housing 18. Suitablefasteners, such as a plurality of bolts 86, are provided for securelyconnecting top cap 22 to outer housing 18.

With continued reference to FIGS. 1 and 2, electric motor 14 isgenerally shown, in this non-limiting example, to include a statorassembly 90, a rotor unit 92, and a sleeve 94. Sleeve 94 has a first endsection 96 engaging end plate section 26 of outer housing 18, a secondend section 98 surrounding a portion of hub 48 on bottom cap 20, and anelongated intermediate sleeve section 100 therebetween. An 0-ring seal102 is provided between annular rim 36 of end plate section 26 and firstend section 96 of sleeve 94. Sleeve 94 is configured to delineate motorchamber 56 into a toroidal stator cavity 56A and a cylindrical rotorcavity 56B. Stator assembly 90 is located within stator cavity 56A andis configured to be non-moveable (i.e. stationary) therein. Rotor unit92 is located within rotor cavity 56B and is configured to be bothrotatable and axially moveable therein, as will be detailed hereinafterwith greater specificity.

Stator assembly 90 includes, in this non-limiting example, a coilwinding 106 and a plurality or stack of plates 108 retained on a statorcage 110. Stator cage 110 in non-moveably mounted to outer housing 18and/or sleeve 94 within stator cavity 56A.

Rotor unit 92 is shown, in this non-limiting example, to include a rotorshaft 114 and a plurality of circumferentially-aligned permanent magnets116 retained by or encapsulated in a rotor shell 118. An annularmagnetic air gap 120 is established between intermediate sleeve segment100 of sleeve 94 and rotor unit 92. The components of rotor unit 92 arefixed to rotor shaft 114 for common rotation about the longitudinalaxis. A first or lower end portion 114A of rotor shaft 114 is disposedin blind bore 58 formed in bottom cap 20 and is supported for rotary andaxial movement therein by a first or lower guide bushing 122 retained inbearing pocket 60. Likewise, a second or upper end portion 114B of rotorshaft 114 extends through throughbore 38 and into impeller cavity 74.End portion 114B of rotor shaft 114 is supported for rotary and axialmovement by a second or upper guide bushing 124 retained in bearingpocket 40 formed in annular boss 34.

Pump unit 16 is shown, in this non-limiting example, to include a rotarypump member, such as an impeller 126, that is rigidly fixed to secondend portion 114B of rotor shaft 114 for rotation within pump pocket 32.Impeller 126 is configured to include a central hub segment 128, a firstor lower rim segment 130 extending radially from hub segment 128, asecond or upper rim segment 132, and a plurality of contoured impellerblades 134 extending between lower rim segment 130 and upper rim segment132. The actual number of impeller blades 134 and their particularcontoured configuration (i.e. profile, shape, thickness, etc.) can beselected to provide the desired flow characteristic for a specific pumpapplication. Upper rim segment 132 is configured to define a first rimsurface 136 that is generally aligned with first flange surface 78 ofvolute interface 76, and define a second rim surface 138 that isgenerally aligned with second flange surface 80.

In accordance with the present disclosure, a rotor/impeller assembly 150(comprised of rotor unit 92, rotor shaft 114 and impeller 126) ismoveable axially with respect to stator assembly 90 and inlet/voluteinterface 76 to provide a means for varying the flow characteristics ofpump 10. In this regard, FIGS. 1 and 2 further illustrate pump 10 toinclude a mechanical biasing arrangement 152 acting between rotor unit92 and a stationary component or portion of pump housing 12. Inparticular, mechanical biasing arrangement 152 is shown, in thenon-limiting example, to include a thrust washer 154 fixed to annularboss 34 (or abutting guide bushing 124) and a biasing member 156 actingbetween thrust washer 154 and an upper portion of rotor unit 92. In thenon-limiting example shown, biasing member 156 is a helical coil springsurrounding rotor shaft 114 and configured to apply a predefined springload (i.e. “preload”) on rotor unit 92 for normally biasing rotor unit92 toward a first position within rotor cavity 56B, as shown in FIG. 1.In this first position, rotor unit 92 is axially offset relative tostator assembly 90. Since impeller 126 is fixed via rotor shaft 114 torotor unit 92, impeller 126 is located in a “retracted” position whenrotor unit 92 is located in its first position. As such, rotor/impellerassembly 150 is defined to be located in a “low flow” position withinpump 10.

As seen in FIG. 1, with rotor/impellor assembly 150 located in its lowflow position, a small clearance “X₁”, is established between a lowersurface 140 of impeller hub 128 and a bottom surface 142 of impellerpocket 32. In contrast, a large clearance “Y₁” is established betweencorresponding interface surfaces 78, 80 and impeller rim surfaces 136,138. The preload provided by biasing member 156 is selected to establishthis offset relationship shown in FIG. 1 between stator assembly 90 androtor unit 92 when the rotor shaft speeds are low so as to increase theclearance gap “Y” between impeller 126 and volute interface 76 tointentionally provide decreased pump efficiency and reduced flow.

In contrast to the arrangement shown in FIG. 1, FIG. 2 illustrates pump10 when rotor shaft 114 is driven at a higher rotary speed.Specifically, when impeller 126 is rotated at higher speeds, a fluidpressure differential across impellor 126 acts to compress biasingmember 156 which permits axial movement of rotor/impeller assembly 150to a “high-flow” position (FIG. 2). With rotor/impeller assembly 150located in its high flow position, rotor unit 92 is located in a secondposition relative to stator assembly 90 and impeller 126 is located inan “extended” position relative to volute interface 76. In its secondposition, rotor unit 92 is axially aligned with stator assembly 90 suchthat a large clearance “X₂” is established between lower surface 140 ofimpeller hub 128 and bottom surface 142 of impeller pocket 32 while,concomitantly, a small clearance “Y₂” is established betweencorresponding interface surfaces 78, 80 and impeller rim surfaces 136,138. The counterforce generated to oppose and overcome the preload ofbiasing member 156 is a result of the pressure differential (AP)generated when impeller 126 is rotated at higher speed.

In one non-limiting example, the clearance gap “Y₁” is in the range of 3to 5 mm at low impellor rotary speeds in the range of 400 to 600 RPM. Incontrast, the clearance gap “Y₂” is in the range of 0.3 to 0.6 mm athigher impellor rotary speeds. FIG. 3 provides a graphical illustrationof the flow vs speed characteristics for a conventional electric waterpump with a fixed rotor/impeller assembly (see line 160) in comparisonto pump 10 of the present disclosure (see line 162). What is evident isthat the reduced efficiency provided by spring-biasing rotary/impellerassembly 150 to its low flow position (FIG. 1) results in reduced flowrates (LPM) at lower pump speeds. The illustration further illustratesthat upon movement of rotor/impeller assembly 150 to its high flowposition (FIG. 2), the flow vs. speed characteristics of pump 10 tend toalign with those of the conventional pump, identified in thisnon-limiting example as point “P”.

Based on the above, the present disclosure provides a unique andnon-obvious variant of an electric water pump 10 that is configured togenerate lower flow at low rotor speeds as well as generate high flow athigher rotor speeds. It is contemplated that the preload applied bybiasing member 156 to rotor unit 92 can be calibrated based on pumpspeed so as to maintain rotor/impeller assembly 150 in its low flowposition until increased pumping efficiency is required.

Referring now to FIGS. 4 and 5, a second embodiment of an electric waterpump 10′ constructed and functional in accordance with the presentdisclosure will be disclosed. Based on the similarity of a majority ofthe components associated with water pumps 10, 10′, common referencenumbers are used with the exception that primed reference numeralsidentified slightly modified components. In general, pump 10′ does notrely on spring-biasing arrangement 152 to provide axial movement ofrotor/impeller assembly 150′, but rather utilizes a magnetic biasingarrangement 152′ provided by an axially-offset magnetic fieldarrangement between rotor unit 92′ and stator assembly 90. Inparticular, rotor unit 92′ is shown equipped with a plurality ofelongated magnets 116′ having extended end segments 116A extendingaxially outwardly from the top portion of rotor unit 92′. Under normalcircumstances, the center of magnets 116′ will naturally align withstator assembly 90, as shown in FIG. 4, so as to locate rotor/impellerassembly 150′ in the low flow position establishing clearance X₁, andY₁, similar to those clearances associated with pump 10 of FIG. 1. Asnoted previously, rotor unit 92′ is located in its first positionrelative to stator assembly 90 and impeller 126 is located in itsretracted position relative to volute interface 76 when rotor/impellerassembly 150 is in its low flow position. This “self-centering” actionat low rotor speeds is caused by the centering behavior of the magneticflux associated with the generated magnetic field.

In contract to FIG. 4, FIG. 5 illustrates pump 10′ when rotor unit 92′is driven at a higher speed which causes the pressure differential (AP)across impeller 126 to forcibly move rotor/impeller assembly 150′ in anupward direction to its second or extended position, therebyestablishing clearances X₂, Y₂ similar to pump 10 of FIG. 2. Again,rotor unit 92′ is located in its second position relative to statorassembly 90 while impeller 126 is located in its extended positionrelative to volute interface 76. Thus, pump 10′ provides a magneticbiasing arrangement as an option to the mechanical biasing arrangementassociated with pump 10. Line “B” in FIG. 5 identifies the stator'scenter magnetic field aligned with the rotor's center magnetic field.The clearance “D” in FIG. 4 identifies an example amount of magneticoffset between the rotor's center magnetic field and the stator's centermagnetic field.

While pump 10 was illustrated to include a helical coil spring asbiasing member 156 those skilled in the art recognize that other typesand/or combinations of biasing devices configured to normally biasrotor/impeller assembly 150 to its low flow position during lowspeed/low flow operation can be employed. In addition, a combination ofthe spring-biased arrangement 152 of FIGS. 1 and 2 can be integratedwith the magnetic field arrangement 152′ of FIGS. 4 and 5 to provide ahybrid variant of yet another embodiment of an electric water pump thatis within the anticipated scope of this disclosure.

While not expressly shown, those skilled in the art will recognize thatelectric pumps 10, 10′ would be equipped with a controller device whichfunctions to control operation of electric motor 12 and the rotationalspeed of impeller 126. The controller device may include an electroniccircuit board (ECB) electrically connected to stator assembly 90 andwhich can be mounted within pump housing 18.

Referring to FIGS. 6A and 6B, another alternative embodiment of anelectric water pump 10″ is shown which is generally similar to electricwater pump 10 of FIGS. 1 and 2 with the exception that impeller 126″ nowincludes a molded-in sleeve 170 within which end portion 114B of rotorshaft 114 is pressed into. In addition, mechanical biasing arrangement152″ now includes a plurality of stacked wave or spring washers 172,such as Belleville washers, surrounding rotor shaft 114 and beingdisposed between a top portion of rotor unit 92 and thrust washer 154.Otherwise, the structure and function of water pump 10″ is generallysimilar to that of water pump 10. While specific aspects, features andarrangements have been described in the specification and illustrated inthe drawings, it will be understood that various changes can be made andequivalent elements be substituted therein without departing from thescope of the teachings associated with the present disclosure.Furthermore, the mixing and matching of features, elements and/orfunctions between various aspects of the inventive electric water pumpsis expressly contemplated. Accordingly, such variations are not to beregarded as departures from the disclosure and all reasonablemodifications are intended to be within the anticipated scope of thedisclosure.

1. A variable flow electric water pump for use in an engine coolantsystem of a motor vehicle, the electric water pump comprising: a pumphousing defining a fluid chamber and a motor chamber, said fluid chamberincluding a fluid inlet and a discharge outlet for providing flow of acoolant through said fluid chamber; an electric motor disposed withinsaid motor chamber of said pump housing and including a stationarystator assembly and a rotor unit having a rotor shaft supported forrotation about a longitudinal axis and extending into said fluidchamber; a pump member fixed to said rotor shaft for rotation in saidfluid chamber and operable to pump coolant from said fluid inlet to saiddischarge outlet; and a biasing arrangement for normally locating saidrotor unit in a first position that is axially offset relative to saidstator assembly for locating said pump member in a retracted positionwithin said fluid chamber to provide a low flow characteristic betweensaid fluid inlet and said discharge outlet when said pump member isrotatably driven by said rotor shaft at a low rotor speed; whereinrotation of said impeller at a high impeller speed causes said rotorunit to move into a second position axially aligned with said statorassembly and causes said pump member to move into an extended positionwithin said fluid chamber to provide a high flow characteristic betweensaid fluid inlet and said discharge outlet.
 2. The electric water pumpof claim 1, wherein said biasing arrangement is a mechanical biasingarrangement including a biasing member configured to exert a preload onsaid rotor unit.
 3. The electric water pump of claim 2, wherein saidbiasing member is a coil spring disposed between a portion of said pumphousing and said rotor unit.
 4. The electric water pump of claim 1,wherein said biasing arrangement is a magnetic biasing arrangementincluding a plurality of magnets extending axially outwardly from saidrotor unit and operable to align the center of a magnetic fieldassociated with said rotor unit with the center of a magnetic fieldassociated with said stator assembly for locating said rotor unit in itsfirst position.
 5. The electric water pump of claim 1, wherein saidrotor shaft is axially moveable relative to said pump housing and has afirst end slideably and rotatably supported by a first guide bushing anda second end slideably and rotatably supported by a second guidebushing.
 6. The electric water pump of claim 1, wherein said pumphousing includes an interface between said fluid inlet and said fluidchamber defining a flange surface, wherein said pump member is animpeller having an outer rim surface aligned with said flange surface ofsaid pump housing, wherein a large clearance gap is established betweensaid outer rim surface of said impeller and said flange surface of saidpump housing when said impeller is located in its retracted position,and wherein said large clearance gap is configured to decrease thecoolant flow rate between said fluid inlet and said discharge outlet. 7.The electric water pump of claim 6, wherein a small clearance gap isestablished between said flange surface of said pump housing and saidrim surface of said impeller when said impeller is located in itsextended position, and wherein said small clearance gap is configured toincrease the coolant flow rate between said fluid inlet and saiddischarge outlet.
 8. The electric water pump of claim 7, wherein apressure differential established across said impeller in response toincreasing impeller speed is operable to cause said impeller to movefrom its retracted position into its extended position, and wherein suchaxial movement of said impeller causes concurrent axial movement of saidrotor unit relative to said stator assembly from its first position intoits second position.
 9. The electric water pump of claim 1, wherein apressure differential established across said pump member in response toincreasing rotor unit speed is operable to cause said pump member tomove from its retracted position into its extended position, and whereinsuch axial movement of said pump member causes concurrent axial movementof said rotor unit relative to said stator assembly from its firstposition into its second position.
 10. A variable flow electric waterpump for pumping a coolant in an engine coolant system of a motorvehicle, comprising: a pump housing defining a fluid chamber and a motorchamber, said fluid chamber including a fluid inlet, a discharge outletand a pumping cavity providing fluid communication between said fluidinlet and said discharge outlet; an electric motor disposed in saidmotor chamber of said pump housing and including a sleeve delineatingsaid motor chamber into a stator cavity and a rotor cavity, a stationarystator assembly located in said stator cavity, and a rotor unit locatedin said rotor cavity and being supported therein for rotation about alongitudinal axis for axial translation along said longitudinal axis; animpeller located in said pumping cavity and fixed for rotation and axialtranslation with said rotor unit; a housing mechanism disposed betweensaid rotor unit and said pump housing and configured for normallybiasing said rotor unit toward a first axial position relative to saidstator assembly which locates said impeller in a retracted positionwithin said pumping cavity to establish a low flow characteristic forthe coolant supplied from said fluid inlet to said discharge outlet inresponse to said impeller being driven by said rotor unit at a lowimpellor speed; wherein rotation of said impeller at a high impellorspeed counteracts the biasing applied to said rotor unit and causes saidrotor unit to move toward a second axial position relative to saidstator assembly which locates said impeller in an extended positionwithin said pumping cavity to establish a high flow characteristic forthe coolant supplied from said fluid inlet to said discharge outlet inresponse to said impeller being driven by said rotor unit at a highimpellor speed.
 11. The electric water pump of claim 10 wherein saidrotor unit is offset relative to said stator assembly in its firstposition, and wherein said rotor unit is aligned relative to said statorassembly in its second position.
 12. The electric water pump of claim 10wherein said biasing mechanism includes a biasing spring configured toexert a preload on said rotor unit for biasing said rotor unit towardits first position.
 13. The electric water pump of claim 10 wherein saidbiasing mechanism is a magnetic arrangement including a plurality ofmagnets mounted to said rotor unit and operable to align a magneticfield established between said stator assembly and said motor unit whensaid rotor unit is located in its first position.
 14. The electric waterpump of claim 10 wherein said rotor unit includes a rotor shaftsupported for sliding axial translation in said pump housing and havingan end portion fixedly secured to said impeller.
 15. The electric waterpump of claim 10 wherein a pressure differential established across saidimpeller in response to increasing the impeller speed is operable tocause said impeller to move from its retracted position into itsextended position which causes said rotor unit to move from its firstposition into its second position relative to said stator assembly. 16.The electric water pump of claim 15 wherein an interface between saidfluid inlet and said pumping cavity defines an annular flange surface,wherein said impeller is configured to include a rim surface alignedwith said flange surface of said pump housing, and wherein a largeclearance gap is established between said flange surface and said rimsurface when said impeller is located in its retracted position, saidlarge clearance gap configured to decrease a flow rate of the coolant.17. The electric water pump of claim 16 wherein a small clearance gap isestablished between said flange surface and said rim surface when saidimpellor is located in its extended position, said small clearance gapconfigured to increase the flow rate of the coolant.